1INDUSTRIAL USEFUL FIELD
This invention relates to a control hydraulic circuit for a speed change clutch applied, for example, to a power transmission of a rough terrain crane etc.
2PRIOR ART
In case of a power transmission such as that of a rough terrain crane etc., for example, in which speed change stages of three forward speeds and one reverse speed are incorporated, clutches acting on respective speed change stages are installed separately, and each clutch actuates each speed change stage independently: a control hydraulic circuit controlling each clutch has conventionally been designed as diagrammatically illustrated by FIG. 2. Namely, in FIG. 2, when a control signal is given to a solenoid for a first spring return type solenoid valve 51. The first solenoid valve 51 actuates to feed pilot pressure oil from a hydraulic power supply through a regulator valve and the first solenoid valve 51 to a first control valve 52. Thereby, the first control valve 52 works, driving pressure oil is fed through the regulator valve, a modulator valve 53 and the first control valve 52 to a reverse clutch 54, and the reverse clutch 54 is actuated to change the speed stage to the reverse speed. Further, when the control signal is given to a solenoid of a spring return type second solenoid valve 56, the second solenoid valve 56 actuates to feed pilot pressure oil from the hydraulic power supply through the regulator valve and the second solenoid valve 56 to a second control valve 57. Thereby, the second control valve 57 works, the driving pressure oil is fed through the regulator valve, the modulator valve 53 and the second control valve 5? to a third-speed clutch 58, and the third speed clutch 18 is actuated to change the speed stage to the third speed. Further, when the control signal is given to a solenoid of a spring return type third solenoid valve 60, the third solenoid valve 60 actuates to feed the pilot pressure oil from the hydraulic power supply through the regulator valve and the third solenoid valve 60 to a third control valve 61.
Thereby, the third solenoid valve 61 works, the driving pressure oil is fed through the regulator valve, the modulator valve 53 and the third control valve 61 to a second-speed clutch 62, and the second-speed clutch 62 is actuated to change the speed stage to the second speed. Further, when the control signal is given to a solenoid of a spring return type fourth solenoid valve 64, the fourth solenoid valve 64 actuates to feed the pilot pressure oil from the hydraulic power supply through the regulator valve and the fourth solenoid valve 64 to a fourth control valve 65. Thereby, the fourth control valve 65 works, the driving pressure oil is fed through the regulator valve, the modulator valve 53 and the fourth control valve 65 to a first-speed clutch 66. and the first-speed clutch 66 is actuated to change the speed stage to the first speed. A spool of the first control valve 52 is connected to a spool of the second control valve 57 by a spring 68. Accordingly, under a state where the pilot pressure oil is fed to the first control valve 52 to cause the spool of the first control valve 62 to compress the spring 68 for example, the spool of the second control valve 57 can not further compress the spring is even when the pilot pressure oil is fed to the second control valve 57, so that the second control valve does not work. In the same way, under a state where the second control valve 57 is working, the first control valve 52 does not work even when the pilot pressure oil is fed to the first control valve 52. Further, a spool of the third control valve 61 is connected to a spool of the fourth control valve 65 by a spring 69. Accordingly, under a state where the pilot pressure oil is fed to the third control valve 61 to cause the spool of the third control valve 61 to compress the spring 69 for example, the spool of the fourth control valve 65 can not further compress the spring 69 even when the pilot pressure oil is fed to the fourth control valve 65, so that the fourth control valve 65 does not work. In the same way. Under a state where the fourth control valve 65 is working, the third control valve 61 does not work even when the pilot pressure oil is fed to the third control valve 61.
In the above-mentioned structure, however, the first control valve 52 does not work simultaneously with the second control valve 57 and the third control valve 61 does not work simultaneously with the fourth control valve 65, so that the reverse clutch 54 does not work simultaneously with the third-speed clutch 58 and the second speed clutch 61 does not work simultaneously with the first-speed clutch 66. Therefore, there are possibilities that the reverse clutch 54 works simultaneously with the second-speed clutch 62 or the first-speed clutch 66, and that the third-speed clutch 58 works simultaneously with the second-speed clutch 62 or the first-speed clutch 60. In this case, a so called locking state occurs wherein transmission of power becomes impossible. There.+-.ore, seizure of clutch or breakage of other components have sometimes arisen. In case where the control signal is kept given to the second solenoid valve 56 even after the control signal has been given to the third solenoid valve 60 due to failure of a control unit or the spool of the second control valve 57 can not return because dust adheres to the spool, when the speed stage is shifted down from the third speed to the second-speed, for example; both the third-speed clutch 58 and the second-speed clutch 62 work simultaneously each other to cause the locking state.